The knee joint essentially operates under compression under the effect of gravity. Its movement features a primary degree of freedom, which is flexoextension (normally in a 160° arch) and a secondary degree of freedom which is rotation about the longitudinal axis of the leg. The latter secondary rotation is present only when the knee is bent since, when the knee is extended, the tibia becomes locked against rotation with the femur. Some mechanical play in the joint allows extra lateral movement when the knee is slightly bent but this small degree of freedom disappears when the knee is fully stretched except in some pathological cases.
Since knees have to satisfy two diverse features, which are extensive mobility when bent more than a certain angle and high stability when straight and the knee has to support the body weight with long lever arms, it is vulnerable to articular fractures and other damage.
Flexoextension, which is the main degree of freedom of the knee, is conditioned by a joint of the troclear type because of the convex shape in both directions of the two (lateral and medial) femoral condyles. Hence, the knee is a dicondylear joint in anatomical terms and a specific troclear joint in mechanical terms.
On the tibial side, the surfaces are shaped conversely over dual parallel concavely landings, the glenoids, separated by a blunt anteroposterior crest where the tibial crown is located. The blunt crest fitting into the inter-condylear cleavage prevents axial rotation in extension.
From the functional point of view, the knee joint includes two joints, the femorotibial joint and the femoro-patelar joint. The former is formed by the condyles clasping onto their glenoids in a way such that the tibial crown fits into the intercondylean cleavage. The femoropatelar is formed by two slopes of the articular surface of the knee-cap with two faces of the femoral troclea, such that the vertical blunt crest couples into the troclear cleavage.
Just from the flexoextension point of view and in a first approach only, one may imagine the knee joint like a dicondylean knee surface sliding over dual matched concave landings. However, reality is quite more complex.
When the condyle and glenoid bearing surfaces are subject to excessive or uneven wear, the joint starts operating badly, the surrounding soft tissue may swell, the area becomes painful, knee movement becomes acutely restricted and so does the amount the knee is able to bend. As an alternative to replacing the entire knee by an orthopedic joint, unicompartimental knee surgery was suggested around 1970, consisting in replacing the bearing surfaces of the damaged compartment, either the medial (internal) compartment or the lateral (external) compartment, regardless of the other, or both. Aside from being surgically less invasive, this treatment sacrifices less healthy bone matter and retains the femoro-patelar joint and the ligament structure of the knee, in addition to the colateral compartment when it is in adequate anatomical and functional conditions.